Studies of intracardiac receptors whose afferent nerve fibers traverse the sympathetic nerve tracts indicate that the receptors are widely distributed in both atria, both ventricles, the aorta, and the pericardium. Within the heart, receptors are located within the epicardium, endocardium, interventricular septum, and individual papillary muscles. The sympathetic afferent nerves conducting impulses from these receptors are of two types: large A type fibers (conduction velocity 7-15 m/sec) and smaller unmyelinated C fibers (conduction velocity less than 1 m/sec). Intracardiac receptors respond to changes in intracardiac pressure and volume and, in some instances, may respond to wall or valve motion. An increase in sympathetic afferent nerve activity related to changes in ventricular segment length was observed during acute coronary artery occlusion. Electrical stimulation of thoracic visceral afferent fibers produced changes in respiratory frequency, changes in heart rate and force of contraction, and changes in renal vascular resistance. BIBLIOGRAPHIC REFERENCES: Purtock, R.V., E.J. Zuperku, S.R. Peters, R.L. Coon, and J.P. Kampine. Response of left ventricular mechanoreceptors to changes in pressure and muscle length. Proc. Soc. Exp. Biol. Med., 1977, in press. Purtock, R.V., J.H. von Colditz, J.L. Seagard, F.O. Igler, E.J. Zuperku, and J.P. Kampine. Reflex effects of thoracic sympathetic afferent nerve stimulaton on the kidney. Am. J. Physiol., 1977, in press.